Wednesday, June 16, 2004

Hack #6:Checking for a valid object instance

I just answered a question in a newsgroup, by simply copying-and-pasting an old post of mine. Reuse is nice, so I decided to put it in my blog as well. Here it is: "Just for the fun of it, I've made some routines to check if a pointer points to a valid object instance. This relies on some types and routines from Ray Lischner's "Secrets of Delphi 2" (the S_VMT unit). Note that this code is D3 specific. Small changes are needed for D1/D2. [Right :)]

uses 
S_VMT;

function ValidPtr(P: pointer; Size: Cardinal): boolean;
begin
Result := not IsBadReadPtr(P, Size);
end;

function ValidObjType(Obj: TObject; ClassType: TClass): boolean;
begin
Result := Assigned(Obj) and
ValidPtr(Pointer(Obj), SizeOf(TObject)) and
ValidPtr(Pointer(Obj), ClassType.InstanceSize);
end;

type
PClass = ^TClass;

function ValidPShortString(S: PShortString): boolean;
begin
Result := ValidPtr(S, SizeOf(Byte)) and
ValidPtr(S, Ord(S^[0])) ;
end;

function ValidClassParent(ClassParent: PClass): boolean;
begin
if ClassParent = nil then
Result := true
else
if ValidPtr(ClassParent, SizeOf(ClassParent^)) then
Result := (ClassParent^ = nil) or ValidClassType(ClassParent^)
else
Result := false;
end;

function ValidClassType(ClassType: TClass): boolean;
var
Vmt: PVmt;
begin
Vmt := GetVmt(ClassType);
Result := ValidPtr(Vmt, SizeOf(Vmt^)) and
(Vmt^.SelfPtr = ClassType) and
ValidPShortString(Vmt^.ClassName) and
ValidClassParent(PClass(Vmt^.ClassParent)) ;
end;

function ValidObj(Obj: TObject): boolean;
begin
Result := Assigned(Obj) and
ValidPtr(PClass(Obj), SizeOf(TClass)) and
ValidClassType(Obj.ClassType) and
ValidPtr(Pointer(Obj), Obj.InstanceSize);
end;

This is probably not foolproof, but it should work in most instances. It works by checking for valid pointers using the Win32 API IsBadReadPtr, checking that the VMT-pointer for the given object is valid. Using this code is not recommended as an alternative to setting instance pointers to nil after freeing them."


Update; I found another old post with a simpler (and probably safer) way of doing it:

function ValidateObj(Obj: TObject): Pointer;
type
PPVmt = ^PVmt;
PVmt = ^TVmt;
TVmt = record
SelfPtr : TClass;
Other : array[0..17] of pointer;
end;
var
Vmt: PVmt;
begin
Result := Obj;
if Assigned(Result) then
try
Vmt := PVmt(Obj.ClassType);
Dec(Vmt);
if Obj.ClassType <> Vmt.SelfPtr then
Result := nil;
except
Result := nil;
end;
end;

Notice that this version is specific to D6 and D7 (IIRC). Other versions might need to update the hardcoded magic number (17).


Update II [June 22, 2007]:
Note that Pierre's "new" FastMM (and thus the D2006/2007 MM) reuses memory blocks more aggressively than the old MM, so you may be false positives from my function if the instance pointer has been freed and just happens to be reallocated by a different object instance (of potentially a different object type).


As always, use this hack with care and a grain of salt or two.

4 comments:

Kostas Terzides said...

Hallvard, does the magic number 17 apply also to D2005? Some simple test I made showed that this is indeed the case, but I'd like to confirm it.

PS: Excellent tip, thanks for this.

Hallvard Vassbotn said...

Yes, 17 should still work in D2005. It only changes if the VMT table for a class changes (new TObject virtual methods, for instance). And that hasn't changed since D5, I think.

Alister Christie said...

I'm havinig trouble reading the code as none of the line breaks seem to be present - was this intended or is it just me ;-). Of course even so, the code would still compile (except for the comments).

Hallvard Vassbotn said...

Alister, no that is not intended. I see it as well. Code in other articles work fine - I'll check more and fix this.

Thanks for spotting it!



Copyright © 2004-2007 by Hallvard Vassbotn